This application claims priority to Japanese Patent Application Number JP2002-015078 filed Jan. 24, 2002, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor device with a charge-storing unit, to a solid-state imaging device such as a charge-coupled device (CCD) image sensor or a complementary metal-oxide semiconductor (CMOS) image sensor, and to a method for making the semiconductor device or the solid-state imaging device.
2. Description of the Related Art
FIG. 4 is a cross-sectional view showing an example layer structure around a pixel unit of a conventional CCD solid-state imaging device.
The solid-state imaging device includes an epitaxial layer 11 formed on an N-type silicon substrate 10. The epitaxial layer 11 includes a photosensor unit (photodiode) 20 that functions as an imaging pixel and a CCD vertical transfer unit 30. A dotted line xcex1 indicates the border between the epitaxial layer 11 and the substrate 10.
The vertical transfer unit 30 is formed as a strip extending in a direction perpendicular to the plane of paper in FIG. 4. A plurality of vertical transfer units 30 is aligned at a regular interval.
Each photosensor unit 20 is formed as a dot that constitutes a two-dimensional matrix. The photosensor units 20 generate signal charges by photoelectric conversion of light incident on the substrate surface and store the generated signal charges.
Referring again to FIG. 4, a transfer electrode 40 is disposed in a region directly above the vertical transfer unit 30 and at the top of the epitaxial layer 11. The transfer electrode 40 is separated from the vertical transfer unit 30 by an insulating film (not shown) therebetween. Each of the transfer electrodes 40 is formed as a strip extending in a direction parallel to the plane of paper of FIG. 4. The transfer electrodes 40 are sequentially aligned in a direction perpendicular to the plane of the paper of FIG. 4.
A vertical-transfer clock pulse is applied to the transfer electrode 40 so as to sequentially transfer the signal charges read out from the photosensor unit 20 to the vertical transfer unit 30. The transferred signal charges are output to a horizontal transfer unit (not shown).
The horizontal transfer unit that received the signal charges from the vertical transfer unit 30 transfers the signal charges in the horizontal direction, i.e., the direction parallel to the plane of the paper of FIG. 4, so as to output the signal charges to a charge-detection amplifier (not shown). The charge-detection amplifier converts the signal charges to voltage signal or current signal and outputs the converted signals.
A light-shielding film 50 is formed to cover each transfer electrode 40. The transfer electrode 40 is separated from the light-shielding film 50 by an insulating film (not shown) therebetween.
An opening 52 is formed between the light-shielding films 50 to expose the light-receiving surface of the photosensor unit 20. While light enters the photosensor unit 20 via the opening 52, the light-shielding film 50 inhibits light from entering the sections other than the photosensor unit 20.
An on-chip-lens (OCL) 60 for condensing incident light is formed above the light-shielding film 50.
An embedded transfer channel 32 and a second P-type well region (2PW) 34 surrounding the transfer channel 32 are formed in the epitaxial layer 11. The transfer channel 32 and the second P-type well region (2PW) 34 form a transfer path for the signal charges and thus function as the vertical transfer unit 30.
An overflow barrier (OFB) layer that allows a vertical overflow structure is formed under the photosensor unit 20. The OFB layer is a potential barrier prepared by forming a first P-type well region (1PW, shaded region in FIG. 4) 70 in the silicon substrate 10.
The photosensor unit 20 consists of an upper layer, which is a P-type impurity region, and a lower layer, which is an N-type impurity region. The region around the interface between the N-type impurity region and the OFB layer is a depletion region. The charges generated in and around the depletion region flow into the photosensor unit 20 via the depletion region.
Accordingly, in order to improve the sensitivity of the photosensor unit 20, the depletion region must be extended over a wider range. In order to extend the depletion region, the position of the first P-type well region (1PW) 70 in the substrate must deep.
Conventionally, in order to form the first P-type well region 70 at a deep position, boron ions for forming P-type well region 70 are first implanted in the silicon substrate 10, and the epitaxial layer 11 of N-type is then formed on the implanted silicon substrate 10.
To be more specific, referring to FIG. 5A, a resist mask with openings is placed on the N-type silicon substrate 10 (first substrate), and ions of a P-type impurity are implanted into the N-type silicon substrate 10 to form the first P-type well region 70. As shown in FIG. 5B, a lightly doped N-type epitaxial layer (second substrate) is then formed, and, subsequently, a transfer channel, a photosensor, and the like are formed in the epitaxial layer.
However, growing of the N-type epitaxial layer after ion plantation has the following problems.
First, a high temperature of, for example, approximately 1,100xc2x0 C. or more is necessary to form the epitaxial layer 11. Because of the high temperature, boron ions for forming the first P-type well region 70 implanted into the N-type silicon substrate 10 diffuse externally. As a result, as shown in FIG. 5C, the distribution of the boron concentration in the first P-type well region 70 becomes more spread compared with the boron distribution concentration immediately after the implantation.
Since both the substrate 10 and the epitaxial layer 11 are N-type, the difference in concentration between the substrate and the epitaxial layer contributes as a p-type impurity. However, the amount of the difference is easily changed according to the growth conditions of the epitaxial layer.
For example, when the process time is long, a significantly large amount of boron ions are driven out by external diffusion. Moreover, the n-type impurity is also driven out due to external diffusion. External diffusion of impurities become significant and the amount of impurities becomes highly unpredictable as the temperature is increased.
Moreover, external diffusion continues even during formation of the epitaxial layer. Since impurity ions also diffuse into the growing epitaxial layer, the amount of the impurity ions becomes further unstable.
In order to form the epitaxial layer, SiH4, dichlorosilane, trichlorosilane, or the like that contains phosphorus or arsenic as the N-type impurity is used. The type of impurity for the epitaxial layer is selected depending on the partial pressure of the N-type impurity in the gas, the growth rate, and the process time. The boron ions that have been driven out by external diffusion may be incorporated into the epitaxial layer, thereby degrading the controllability of the amount of the impurity.
Accordingly, the impurity concentration of the first P-type well region 70 and that of the epitaxial layer 11 are difficult to control.
The instability in impurity concentration in these regions results in instability in formation of the depletion region. Moreover, it also results in varying of the voltages applied to the substrate during accumulation of saturating signal charges and in varying of high voltages applied to the substrate for flushing the charges in the substrate direction, i.e., voltages for operating electronic shutters.
Furthermore, because the impurity concentration in the photosensor is widely distributed, the size of the depletion region change accordingly. Since incident light reaches a different depth depending on the wavelength, there is a problem in that the sensitivity varies depending on the wavelength.
As described above, according to known methods, layer structures that are sensitive to long wavelengths cannot be stably formed, and the yield is difficult to improve. Moreover, not every imaging device has the designed structure.
The reason for forming the epitaxial layer subsequent to the formation of the first P-type well region 70 is as follows. The ion implantation apparatus used to implant boron ions can only perform implantation at a low injection energy; accordingly, ion implantation at a deep position of the substrate is difficult.
However, recently, ion implantation apparatuses with high implantation energy are available. They may be used in manufacturing of solid-state imaging devices such as those described above. Since the position of forming the OFB layer directly affects the characteristics of the photosensor, ion implantation must be performed in highly stable conditions.
It is an object of the present invention to eliminate adverse affects of the temperature for forming an epitaxial layer on formation of a first-P-type well region at the deep position of the substrate. According to the present invention, the first P-type well region can be formed at a desired position by adequate ion implantation so as to effectively form an overflow barrier layer. In this manner, a highly sensitive solid-state imaging device or a semiconductor device can be formed. A method for making the solid-state imaging device or the semiconductor device is also provided.
To achieve these goals, an aspect of the present invention provides a solid-state imaging device comprising a substrate composite comprising a first substrate and a second substrate formed on the first substrate, a photosensor for generating signal charges in response to incident light, the photosensor being formed in the second substrate, and a barrier layer formed under the photosensor. The barrier layer is formed by implanting impurity ions from the surface of the second substrate at an energy exceeding 3 MeV. According to this structure, the barrier layer does not suffer from problems arising from the formation of the second substrate.
Another aspect of the present invention provides a method for making a solid-state imaging device comprising a first step of preparing a substrate composite comprising a first substrate and a second substrate on the first substrate, a second step of implanting impurity ions from the surface of the second substrate at an energy exceeding 3 MeV so as to form a barrier layer, and a third step of forming a photosensor in the second substrate. In this manner, the process of forming the second substrate does not affect the barrier layer.
Yet another aspect of the present invention provides a semiconductor device comprising a substrate composite comprising a first substrate and a second substrate formed on the first substrate, a charge-storing region formed in the second substrate, and a barrier layer formed under the charge-storing region. The barrier layer is formed by implanting impurity ions from the surface of the second substrate at an energy exceeding 3 MeV. According to this structure, the barrier layer does not suffer from problems arising from the formation of the second substrate.
Yet another aspect of the present invention provides a method for making a semiconductor device comprising a first step of preparing a substrate composite comprising a first substrate and a second substrate on the first substrate, a second step of implanting impurity ions from the surface of the second substrate at an energy exceeding 3 MeV so as to form a barrier layer, and a third step of forming a charge-storing unit in the second substrate. In this manner, the process of forming the second substrate does not affect the barrier layer.